The invention relates to a process for printing and/or coating of a substrate, of paper or cardboard, a device for executing and a device for printing and/or coating a substrate, especially of paper or cardboard.
In one known process of electrostatic printing, a latent electrostatic image is developed by charged toner particles. The latter are transferred to an image receiver substrate, which is hereinafter also called a substrate for short. Then the developed image which has been transferred to the substrate is fixed by the toner particles being warmed or heated and melted, and optionally the substrate being heated. To melt the toner particles, contact methods are often used, in which the toner particles are brought into contact with corresponding hot rollers or drums. To fix the toner which has been transferred for example to paper, there are furthermore heaters and processes which work without contact in which the toner particles are melted for example using heat/microwave radiation or hot air so that they stick to the paper.
In the printing or copying process in which the image applied to the substrate or the coating is developed by a toner, the gloss of the fixed toner does not follow the gloss of the paper, as is the case for example in offset printing. This applies both to contactless and also contact-making toner fixing processes. The reason for this is the greater layer thicknesses of the toners compared to the color layers produced in the offset printing process. The relatively thick toner layers fill the porosity of the paper, by which a smoothed surface structure and a certain gloss of the toner layer are formed.
In many cases it is desirable to adapt the gloss of the substrate surfaces covered with one or more fixed toner layers to the gloss of the uncovered surfaces of the substrate. For this purpose there have been gloss variation mechanisms, for example in the form of a calendar, which have a surface which acts with a predetermined force on at least one fixed toner layer or a toner layer in the course of being fixed, in order to vary the gloss of the fixed toner layer. One disadvantage of the known gloss variation mechanism is that the surface acting on the toner layer must be replaced by a surface with another structure when the aforementioned matching of the gloss of the fixed toner to the gloss of the uncovered surfaces of the substrate for different toner types and/or different substrates is to be done. The replacement of the corresponding surface of the gloss variation mechanism or the replacement of the entire gloss variation mechanism is however time-consuming and leads to the undesirable machine downtimes.
The object of the invention is to devise a process and a device with which the gloss of at least one toner layer fixed on the substrate can be varied without gloss variation mechanisms or parts thereof having to be replaced. Another object of the invention is to improve the printing or copying quality.
To achieve this object, a process is proposed which relates to printing and/or coating of a substrate, especially of paper or cardboard, at least one toner layer being transferred to the substrate and fixed on it. For example, in color printing the toner image can have four differently colored toner layers, conventionally one of the toner layers at a time being black, yellow, magenta and cyan. The liquid or dry toner has at least one polymer and color pigments and other constituents. When at least one toner layer is fixed, the polymer chains are crosslinked, by which a fixed toner layer is formed which is also called a polymer layer. The surface roughness and thus the gloss of this polymer layer depend largely on the density of the toner particles, their theological properties, and the melting process used. To match the gloss of the developed toner layer for example to the gloss of the uncovered surfaces of the substrate, there is a gloss variation device which has a surface which acts upon at least one fixed toner layer or one which is being fixed. To enable matching of the gloss, for example to various substrates, without replacing the gloss variation device or parts thereof, in the process it is provided that depending on the gloss to be achieved the force with which the surface of the gloss variation device acts on at least one toner layer and/or at least one property of the surface of the gloss variation device is changed. In this way, matching of the gloss in many cases can be done even during operation of the corresponding device, by which machine downtimes can be avoided and the throughput can be increased.
Changing at least one property of the surface of the gloss variation device can comprise changing the roughness of the surface of the gloss variation device. The roughness is determined especially by the distribution and the height of the elevations on the surface of the gloss variation device. In this connection it is possible to change the intervals of the elevations which determine the structure of the surface of the gloss variation device. Likewise, embodiments are conceivable in which the height of the elevations is changed, and changes the force with which the surface acts on at least one toner layer, preferably being chosen in this case such that the elevations for varying the gloss extend entirely into at least one toner layer.
One possibility for changing the structure of the surface of the gloss variation device is to elastically change the dimensions of the surface. In this connection rotation of the elastic material, which forms the surface of the gloss variation device leads to the distances between the individual elevations increasing, like the dimensions of the elevations themselves. When the forces which cause stretching of the elastic material which, forms the surface of the gloss variation device are reduced, the distances between the elevations and the dimensions of the elevations themselves decrease, again as a result of the elasticity of the material. The change in the dimensions of the surface of the gloss variation device can take place for example by changing the diameter of the drum, which forms the surface of the gloss variation device. Alternatively or additionally, it can be provided that the distance between and/or the length of at least the two drums around which an elastic belt is guided which forms the surface of the gloss variation device is changed.
In one embodiment of the process, it is provided that the gloss variation device is formed by a fixing device which is designed for fixing at least one toner layer. In this case the surface of the gloss variation device can be formed for example by the surface of a heated drum which is a component of the fixing device. The surface of the gloss variation device then acts on at least one toner layer in the course of fixing. This embodiment of the process is characterized by the space requirement being hardly increased at all relative to known devices without the gloss variation device.
In another embodiment of the process it is provided that there is a gloss variation device separate from the fixing device which is necessary for fixing at least one toner layer. In this case the gloss variation device can carry out final processing of the at least essentially already fixed toner layer. This can be advantageous for example in cases in which the gloss of at least one toner layer after fixing is acquired by measurement in order to match the final processing carried out by the glass variation device to the actual conditions.
In the process, it can furthermore be provided that via changing the force with which the surface of the gloss variation device acts on at least one toner layer, elevations of the surface of the gloss variation device which taper towards its free end can be pressed to different degrees into at least one toner layer. The elevations of the surface of the gloss variation device can have for example a pyramidal or conical shape. When a surface which is provided with these elevations acts with greater force on at least one toner layer, in this toner layer deeper notches are formed which lead to a reduction in the distances of the notch edges compared to the case in which this surface of the gloss variation device acts with a lower force on at least one toner layer.
In addition, in the process it can be provided that via changing the force with which the surface of the gloss variation device acts on at least one toner layer a different number of elevations of the surface of the gloss variation device is brought into contact with at least one toner layer. For this purpose, for example, it can be provided that the surface of the gloss variation device has elevations which have a different height. In this case, by the action of the surface with a greater force a larger number of elevations come into contact with at least one toner layer.
Changing at least one property of the surface of the gloss variation device can furthermore comprise application of particles to the surface of the gloss variation device and/or alignment of particles on the surface of the gloss variation device. The particles can be for example electrically charged particles, ferroelectric particles or magnetic particles. In this way it is for example possible to provide different printed surfaces of a picture with different gloss properties by digitally controlling the distribution of particles on the surface of the gloss variation device according to the gloss properties to be achieved.
In this connection, one embodiment of the process calls for the adhesion forces between the particles on the surface of the gloss variation device and the surface of the gloss variation device to be greater than the adhesion forces between the particles on the surface of the gloss variation device and at least one toner layer or the substrate. In this way it can be ensured that the particles, at least for the most part, are not transferred to the toner layer or to the substrate, but remain on the surface of the gloss variation device. In this case it is possible to re-align the particles accordingly.
Another embodiment of the process in this connection calls for the adhesion forces between the particles on the surface of the gloss variation device and the surface of the gloss variation device to be smaller than the adhesion forces between the particles on the surface of the gloss variation device and at least one toner layer or the substrate. In this way it is ensured in this embodiment that the particles, at least for the most part, are transferred to at least one toner layer or to the substrate. In this case, particles which extend over the surface of at least one toner layer form elevations, with a density which can be used to change the gloss within wide ranges.
The application of particles to the surface of the gloss variation device and/or the alignment of the particles on the surface of the gloss variation device can for example comprise one or more of the following process steps:
powder coating processes,
use of at least one beam of charged particles,
producing and developing a latent electrostatic structure on the surface of the gloss variation device,
producing and developing a latent magnetic structure on the surface of the gloss variation device,
aligning electrically charged particles on the surface of the gloss variation device by controlled electrical fields,
aligning ferroelectric particles on the surface of the gloss variation device by controlled electrical fields,
aligning magnetic particles on the surface of the gloss variation device by controlled electrical fields,
The invention relates furthermore to a device for printing and/or coating of a substrate, especially of paper or cardboard, with a fixing device for fixing of at least one toner layer on the substrate, and with a gloss variation device which acts on at least one fixed toner layer or one toner layer in the course of being fixed in order to influence the gloss of the fixed toner layer. In this printing device it is provided that the gloss variation device has a surface which acts with variable force on at least one toner layer and/or of which at least one property can be changed to influence the gloss of at least one fixed toner layer. One such printing device which is also suitable for executing the process is characterized in that the gloss of at least one toner layer can be adapted for example to the gloss of the substrate without the gloss variation device, or parts of it having to be replaced. The gloss can therefore be varied during operation of the printing device so that the printing device can prevent downtimes; this enables higher throughput. Furthermore, the printing device enables for example different surfaces of the same sheet to be provided with different gloss properties.
In the printing device it can be provided that the roughness of the surface of the gloss variation device can be changed. When the surface of the gloss variation device acts on at least one toner layer in the manner of an embossing process, thus via the roughness of the resulting toner layer its gloss can be influenced.
The roughness can for example be changed by the dimensions of the surface of the gloss variation device being elastically changed, as was explained in conjunction with the process.
Especially for this purpose, in the printing device it can be provided that the gloss variation device has at least one drum with a variable diameter and/or with a variable length. The drum surface then preferably forms at least in sections the surface of the gloss variation device which acts on at least one toner layer.
Alternatively, it can be provided that the gloss variation device has at least two drums with a variable distance and/or with a variable length. In this case an elastic belt is guided over at least two drums, the belt surface forming at least in sections the surface of the gloss variation device. The dimensions of the elastic belt and thus of the surface of the gloss variation device can thus be changed via changing the distance between the two drums and/or the length of the drums.
In the printing device it can either be provided that the fixing device forms the gloss variation device or that the gloss variation device is separate from the fixing device, reference being made to the corresponding disclosures in conjunction with the process.
The surface of the gloss variation device can have elevations with tapering free ends. If the surface of the gloss variation device acts on at least one toner layer in the manner of an embossing process, the embossed pattern which has been left on at least one toner layer and thus the gloss depend on the force with which at least one toner layer is acted upon.
In addition it can be provided in the printing device that the surface of the gloss variation device has elevations with different height. In this case the number of elevations coming into contact with at least one toner layer depends on the force with which at least one toner layer is acted upon. In this case it can also be provided that all or some of the elevations have tapering free ends.
Furthermore, it can be provided that the printing device has a mechanism for applying particles to the surface of the gloss variation device and/or for aligning particles on the surface of the gloss variation device, reference being made to the corresponding statements in conjunction with the process.
A mechanism for applying particles to the surface of the gloss variation device and/or for aligning particles on the surface of the gloss variation device can comprise for example one or more of the following:
powder coating apparatus,
particle beam generator to produce at least one beam of charged particles,
mechanism for producing and developing a latent electrostatic structure on the surface of the gloss variation device,
mechanism for producing and developing a latent magnetic structure on the surface of the gloss variation device,
mechanism for alignment of electrically charged particles on the surface of the gloss variation device by controlled electrical fields,
mechanism for alignment of ferroelectric particles on the surface of the gloss variation device by controlled electrical fields,
mechanism for alignment of magnetic particles on the surface of the gloss variation device by controlled electrical fields.